The present invention relates to a semiconductor device including a variable gain amplifier circuit.
Recently, there has been proceeding development of multimode RFICs (Radio Frequency Integrated Circuits) which conform to GSM (Global System for Mobile Communications), EDGE (Enhanced Data Rates for GSM Evolution), and WCDMA (Wideband Code Division Multiple Access) which are related art communication standards for cellular phones and LTE (Long Term Evolution) which is a next-generation high-speed data communication standard.
For a transmission system of such a multimode RFIC, transmission power control having a high dynamic range and a high accuracy step defined in the 3GPP (Third Generation Partnership Project) standard is required. Further, it is required to reduce reception band noise to negate the need for a SAW (Surface Acoustic Wave) filter.
Japanese Unexamined Patent Publication No. 2010-273284 (Patent Document 1) discloses a variable gain amplifier circuit having a high dynamic range, high accuracy, and low noise necessary to meet these requirements. Specifically, in the variable gain amplifier circuit in Patent Document 1, 21 amplifiers having gains varying in 6 dB steps are coupled in parallel. The inputs of the amplifiers are coupled to a common input terminal, and the outputs of the amplifiers are coupled to a common load. The amplifiers are selectively put into an operating state in accordance with a control word, thereby controlling the gain of the variable gain amplifier circuit.
Further, the requirements of the multimode transmission system include higher linearity of a transmission amplifier. In order to restrain distortion of an LTE signal having a high PAPR (Peak to Average Power Ratio) within specifications, it becomes necessary to enhance the linearity of the transmission amplifier. Only increasing a power source voltage supplied to the amplifier can suppress the signal distortion but decreases the efficiency of the amplifier.
Envelope tracking is known as one means for achieving an amplifier having high linearity and high efficiency. The envelope tracking is a technique for adjusting a power source voltage supplied to the amplifier in accordance with the amplitude of an input signal (corresponding to the envelope of a signal waveform). By increasing the power source voltage as the amplitude of the input signal increases, it is possible to always use the transistor in a state close to saturation power.
Techniques described in Non-patent Documents 1 and 2 are known as examples in which envelope tracking is applied to a 5-GHz-band CMOS (Complementary Metal Oxide Semiconductor) amplifier. In a circuit configuration described in Non-patent Document 1, an output current detection circuit is added to a related art cascode amplifier. In a circuit configuration described in Non-patent Document 2, an input detection circuit for generating a detection current according to an input power and a bias circuit for adding a feedback current proportional to the detection current to a bias current are added to a related art amplifier.
[Patent Document 1]
    Japanese Unexamined Patent Publication No. 2010-273284[Non-Patent Document 1]    Ueda and four others, “5-GHz-Band CMOS Power Amplifier of Output Current Feedback Type”, Proceedings of the IEICE General Conference, Electronics (1), March 2009, C-2-26, P. 65[Non-Patent Document 2]    Shinjo and three others, “5-GHz-Band CMOS Linear Transmission Driver Amplifier of Input Detection Current Compensation Type”, Proceedings of the IEICE Electronics Society Conference, Electronics (1), September 2009, C-2-36, P. 64